One way of increasing contrast in an image shown on a display, for example a LCD or plasma display, is to apply a gain (>1) to the image data. For example, this may be desired in images where the pixels have nearly the same luminance value, e.g. in pictures with a lot of dark regions. The gain results in higher video levels, i.e. typically a brighter image, and a greater spread amongst the video levels, i.e. an increase of perceived contrast.
Another situation where it may be desired to apply a gain, is when a Dynamic Backlight Algorithm (DBL) is used in an LCD. In LCD's, dark scenes tend to look “bluish” due to leakage of backlight through the LCD panel and a solution to this is to decrease the backlight luminance at such scenes by use of a DBL. When the backlight luminance is reduced, a video gain is required as compensation to accomplish a similar impression of brightness and similar, or even improved perceived contrast as before the reduction of backlight luminance.
Further, the displayable range of video levels typically is limited to a fixed number of different colors, within fixed ranges. For example, each one of the three components in a RGB-representation of a pixel in an image, may be restricted to 8-bits, providing 256 possible video levels (represented by integer values 0-255) for each component (R, G or B).
Corresponding limitations of course also apply in other types of video level representation, or color gamuts, e.g. luminance (Y) in a CMY-representation.
Typically, low values represent darker colors, and high values represent lighter colors, e.g. 0 represents the darkest color, and 255 represents the lightest color.
When a gain applied to the image data results in that the video level of a pixel is out of the displayable range, e.g. exceeds a maximum value such as 255, a known method is to “hardclip” such values, i.e. pixels that, according to the gain, would get a value exceeding the maximum value will instead end up being equal to the maximum value (e.g. 255). A consequence from this is loss of local details; substantial parts of the image may be shown as ‘flat’ regions of highly saturated color (all pixels in the region having the maximum value) and certain details in the original image may be lost.
Thus, by applying a gain to increase the value for some pixels in a region of an image, there is a risk for loss in contrast at other regions due to hardclipping. This often becomes a limiting factor with respect to how much gain that can be applied in practice.
EP0963111A1 discloses a method and apparatus for dynamic contrast improvement in images by use of a dual segment transfer function.
FIG. 1 shows an input to output diagram as disclosed in EP0963111A1, comprising a transfer function for luminance values in units of IRE (Institute of Radio Engineers). When there is no contrast improvement, there is no gain (a 1:1 relation input to output) and the transfer function is simply a “unity gain”-function 10, i.e. a line with slope 1 that intersects the origin of coordinates. The “dual segment” referred to in EP0963111A1, relates to a transfer function 11, represented by the line 11 in FIG. 1, consisting of two segments, a first segment with slope >=1, preferably 1.5, that at an adaptive pivot point 12 changes into a second segment with slope>=1, preferably 1.7. The adaptive pivot point is always located along the unity gain line 12. In FIG. 1, the adaptive pivot point is shown at its upper end position 14 (40,40). The lower end position (7,7), is represented by point 13.
In the method of EP0963111A1, the video frames, or fields, are analyzed in real time. In the first segment, the segment gain (the slope) is adaptive to the dark sample distribution—a higher value is selected if there are fewer dark samples, and a lower value if there is a high number of dark samples.
In the second segment, the segment gain (the slope) is adaptive to the detected peak value, and the “unity gain”-function is selected if the detected peak value is equal to, or higher than, the nominal peak value, else the “second gain” transfer function is used.
However, although the method of EP0963111A1 can be used to improve contrast in both darker and light regions, it can only provide a relatively small gain, thus small improvement of contrast, especially in case of pictures with high numbers of dark samples, e.g. in scenes where dark pixels dominate. This, for example, makes it unsuitable for use in connection with dynamic backlight in LCDs.
Further, although the method of EP0963111A1 can be used to prevent hardclipping in light regions by use of “unity gain” in the second segment, any gain applied in the first segment may result in a need to hardclip in dark regions, i.e., since the transfer function can produce values below 0 (black), the output may need to “clip” these values to black, and hence contrast in dark regions will be lost.